[llvm-dev] ThinLTO status in trunk?

Adve, Vikram Sadanand via llvm-dev llvm-dev at lists.llvm.org
Fri Jul 22 14:35:05 PDT 2016

On Jul 22, 2016, at 4:31 PM, Mehdi Amini <mehdi.amini at apple.com<mailto:mehdi.amini at apple.com>> wrote:

On Jul 22, 2016, at 12:36 PM, Teresa Johnson via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hi Vikram,


I'm not sure what part got committed in the 3.8 timeframe - it looks like that was released back in March?

3.8 was branched in early January though.
It has some of the work-in-progress for ThinLTO, it “could” work in simple cases I guess, but it hasn’t really been tested.
Also, in 3.8, the state was basically what was presented at EuroLLVM, while (as explained in the blog post), we ended up redesigning it significantly.

OK.  I’m not really trying to use 3.8.

In 3.9 we changed the bitcode format and broke backward compatibility: 3.9 won’t be able to read a ThinLTO bitcode generated by 3.8 (this is possible only because “ThinLTO was not supported in 3.8”).

Since 3.9 isn’t quite out yet, is this also true for trunk, then?  Thanks,



A number of fixes have gone in since then so I would stick with a more recent version. The ThinLTO in trunk and (presumably 3.9 which seems to have not yet gone out?) is complete and working with multiple threads using gold.

The distributed build support is also in (which needs support in the build system however), although I am fixing a few bugs right now that popped up in testing our internal apps. Which distributed build system do you use? We have support coming out in Bazel, which is our open sourced distributed build system.

Let me know if you run into any issues or have any other questions!


On Fri, Jul 22, 2016 at 12:15 PM, Adve, Vikram Sadanand via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
First, kudos on the ThinLTO results reported in your blog post — they’re impressive and the system sounds really well engineered.

I’m starting to try it out on a large piece of software and I’d like to make sure I know what to expect.  The blog said it will be available in clang-3.9 but both clang-3.8 and trunk seem to have some degree of support for it.  What is the status of ThinLTO in 3.8 (preferably) or trunk (otherwise)?  Are either of these usable with multiple threads?  What about for a distributed build?



// Vikram S. Adve
// Professor, Department of Computer Science
// University of Illinois at Urbana-Champaign
// vadve at illinois.edu<mailto:vadve at illinois.edu>
// http://llvm.org<https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_&d=CwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=b7uK7dJM4Fx7J_ehsuohEdD-6NdkoLyTwBFfHX-XKcc&m=JW9g_P9abYPtr9wIwNKwtHt0tcKqV8j_7oJ-7R7dd8U&s=K7WpW1rDZu2vzCM633oS96pOLLMpi6Tkn2wbIARNnBw&e=>

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